\Big{|}A_1 \cup A_2 \cup A_3 \Big{|} =\Big{|}A_1 \Big{|} + \Big{|}A_2 \Big{|} + \Big{|}A_3 \Big{|} - \Big{|}A_1 \cap A_2 \Big{|} - \Big{|}A_1 \cap A_3 \Big{|} - \Big{|}A_2 \cap A_3 \Big{|} + \Big{|}A_1 \cap A_2 \cap A_3 \Big{|}